1. Field of Invention
This invention relates to communication interfaces and design methods therefor. More particularly, the invention relates to a reconfigurable telephone line interface having synthesized matching impedances for different frequency bands on connecting telephone lines and a design method therefor.
2. Description of Prior Art
Communication circuits, more particularly, telephone line interface circuits, are required to present a terminating impedance to voice or data signals on connecting cable pairs. The terminating impedance prevents, or at least minimizes, noise on the cable pairs in the form of reflections or echos due to mismatches between the terminating impedance of the interface circuit and the transmission impedance of the cable pair. In most countries, the terminating impedance for cable pair must meet requirements specified by a country's telephone administration. In some countries, the required terminating impedance is different in characteristic for adjacent frequency bands served by the interface circuit. Synthesis of a terminating impedance that facilitates matching the impedance requirements of different countries would simply implementations for the different countries and lead to a single circuit that could be programmed to meet the requirements of the different countries.
Recently, active filters using transconductance amplifiers with feedback have been used to synthesize a terminating impedance. Prior art related to terminating impedances for telephone interface circuits using a transconductance amplifier with feedback for matching the impedance requirement of a connecting telephone line or cable pair includes the following:
U.S. Pat. No. 5,500,894 issued Mar. 19, 1996 discloses a telephone line interface circuit with AC and DC transconductance loops for providing selectable impedances to a cable pair. The '894 patent discloses a transconductance loop with a filter network having a transfer function designed to provide a synthesized terminating impedance which may be programmed to meet the terminating impedance requirements of different telephone systems. However, '094 does not disclose a cascade of sub-filter networks to meet the terminating impedance requirements of different frequency bands on a cable pair for different countries while having little or no effect upon the other bands.
U.S. Pat. No. 4,894,864 issued Jan. 16, 1990 discloses a transconductance loop for synthesizing the terminating impedance of a cable pair. However, there is no partitioning the loop into programmable sub-filter networks providing different terminating impedances that may be required by different countries.
U.S. Pat. No. 4,381,561 issued Apr. 26, 1983 discloses impedance matching for an interface circuit using digital impedance synthesizing by a digital filter and D/A converter in a negative feedback loop to match a full duplex analog telephone line to a digital switching system. The '561 patent does not disclose a transconductance loop for synthesizing analog signals on a cable pair into matching impedances for different frequency bands.
U.S. Pat. No. 4, 359,609 issued Nov. 16, 1982 discloses an interface circuit having a feedback loop, either current or voltage controlled by which the circuit impedance can be easily set to match a telephone circuit. The '609 patent does not disclose partitioning a filter network in the loop to provide synthesized impedances matching different frequency bands on a cable pair.
U.S. Pat. No. 4,351,060 issued Sept. 21, 1982 discloses an automatic equalizing function implemented digitally using a recursive digital filter having programmable coefficients and a transconductance amplifier to provide an output impedance matching a telephone line. The '060 patent does not disclose matching a synthesized impedance to different frequency bands on a cable pair.
U.S. Pat. No. 4,961,219 issued Oct. 2, 1990 discloses synthesing an impedance across a telephone line by detecting a voltage difference across the telephone line and using an RC circuit to develop a pair of impedance output signals matching the impedance of the telephone line circuit. The '219 patent does not disclose synthesizing matching impedances to that of different telephone line circuits.
None of the prior art discloses or suggests an easily reconfigurable terminating impedance for a communication interface using a transconductance amplifier with feedback that provides different synthesized impedances for different frequency bands and, in particular, uses a notched response to separate one frequency band from another frequency band.